Replace FontAwesome font with SVG icons

I tried this for the footer icons as part of https://github.com/jquery/jquery-wp-content/commit/28542b5616895cba4ac4fa41144abd05b5087a04 and that seemed to work quite well. We only use a handful of the icons, and so rather than loading a 23KB font file, we can load a handful of SVG icons. This would also make updates easier as right now it feels a bit scary to change this since it's one file for everything and it's unclear which ones we actually need.

Proposed outline:

• Remove unused icon- utility classes, based on GitHub org-wide search for the class name (excluding false positives, such as from archived repos, and repos containing content that isn't themed by wordpress, such as web-event, irc.jquery.org and bugs.jquery.com, and demos.jquerymobile.com).
• Reduce icon class definitions to only those that are used.
• Replace one icon- with an SVG version.
• Replace the rest
• Remove FontAwesome from repo.